Portable engine working machine and rotary carburetor incorporated therein

ABSTRACT

To improve responsiveness of fuel supply control, a rotary carburetor 100 has a nozzle 8 including a fuel discharge port 8a and a needle 10 disposed coaxially with the nozzle 8 and disposed with a portion inserted into the nozzle 8. The needle 10 can be displaced relative to the nozzle 8 to change an effective area of the fuel discharge port 8a. The rotary carburetor 100 has an electric motor 14 for displacing the needle 10 along an axis, and a drive mechanism component 12 interposed between the electric motor 14 and the needle 10 and converting a rotational movement of the electric motor into a linear movement.

BACKGROUND OF THE INVENTION

The present invention relates to a portable engine working machine and arotary carburetor incorporated therein.

Portable engine working machines specifically include chain saws, bushcutters, hedge trimmers, power blowers, etc. Portable engines are oftenequipped with carburetors.

Portable engine working machines are increasingly computerized, and anexample thereof is a solenoid valve adopted for fuel control (PatentDocuments 1, 2). In Patent Document 1, an engine including a solenoidvalve for fuel control employs a combination of the solenoid valve and abutterfly carburetor. On the other hand, in Patent Document 2, an engineincluding a solenoid valve for fuel control employs a combination of thesolenoid valve and a rotary carburetor.

The butterfly carburetor disclosed in Patent Document 1 has a dischargepart facing an intra-carburetor air-fuel mixture passage, and fuel issucked through this discharge part into an intake passage in thecarburetor. Similarly, the rotary carburetor disclosed in PatentDocument 2 has a nozzle projected into an intra-carburetor mixturepassage, and fuel is sucked through this nozzle into an intake passagein the carburetor. Therefore, the fuel is supplied from the dischargepart to the intake passage by utilizing a negative pressure of theintra-carburetor air-fuel mixture passage. The solenoid valve isdisposed in an intra-carburetor fuel supply passage leading to thedischarge part or the nozzle. It is noted that the rotary carburetordisclosed in Patent Document 2 does not include a needle inserted into atip portion of the nozzle to control a fuel discharge amount. This isspecified in paragraph [0038] of Patent Document 2.

As understood from Patent Documents 1 and 2, the rotary carburetor isemployed as a carburetor incorporated in a portable engine workingmachine in addition to the butterfly carburetor. A basic structure ofthe rotary carburetor has a valve main body rotatable in a carburetormain body, a nozzle arranged coaxially with a rotation axis of the valvemain body, and a needle inserted into the nozzle from a tip of thenozzle. An effective cross-sectional area of an intake passage iscontrolled by rotation of the valve main body. A fuel discharge amountis controlled by movement of the needle relative to the nozzle. InPatent Document 2, as described above, it is proposed that the needle iseliminated to interpose the solenoid valve instead in theintra-carburetor fuel supply passage leading to the needle.

An example of a rotary carburetor incorporated in a portable engineworking machine is disclosed in Patent Document 3. In the rotarycarburetor disclosed in Patent Document 3, a nozzle is stationary in anon-rotatable manner. The nozzle has a fuel discharge port at a tipthereof, and this fuel discharge port has a tapered shape in acircumferential direction. A needle is rotatable in conjunction with avalve main body and rotates around an axis together with the valve mainbody. The needle also has an opening vertically extending in acircumferential surface thereof. The valve main body and the needledescribed above are linked to a throttle lever operated by an operatorfor output adjustment such that the valve main body and the needlerotate around an axis.

When the operator operates the throttle lever, the valve main body andthe needle rotate around an axis. This changes the effectivecross-sectional area of the intake passage, i.e., a throttle valveopening degree. Additionally, the rotation of the needle relative to thestationary nozzle changes an area of an effective fuel outlet formedwhen the opening of the needle coincides with the fuel discharge port ofthe nozzle. Consequently, the rotary carburetor has the fuel dischargeamount mechanically controlled together with the effectivecross-sectional area of the intake passage (the throttle valve openingdegree).

Patent Document 4 discloses a most popular rotary carburetor in theportable engine working machine. In the rotary carburetor of PatentDocument 4, a nozzle is stationary in a non-rotatable manner. Arotatable valve main body is displaceable in the axial direction of thenozzle because of a support pin and a cam surface engaged therewith. Aneedle is integrated with the valve main body. The needle is displacedin the axial direction in conjunction with the axial rotation of thevalve main body and the displacement in the axial direction associatedtherewith. The nozzle has a fuel discharge port on a circumferentialsurface of a tip portion thereof, and the effective opening area of thefuel discharge port is controlled by an advancing/retracting movement ofthe needle inserted into the tip of the nozzle. In other words, the fueldischarge amount is controlled by the relative advancing/retreatingmovement of the needle.

When the operator operates the throttle lever, the valve main bodymechanically linked thereto rotates. An effective cross-sectional areaof the intake passage in the carburetor, i.e., the throttle valveopening degree, changes according to the rotation of the valve mainbody. The rotation of the valve main body induces an axial displacementof the valve main body due to the cam surface. The axial displacement ofthe valve main body is accompanied by a relative displacement of theneedle in the axial direction. On the other hand, since the nozzle isstationary, the effective opening area of the fuel discharge port of thenozzle circumferential surface varies according to the displacement ofthe nozzle in the axial direction.

Patent Document 5 discloses a rotary carburetor applied to a stratifiedscavenging engine. The stratified scavenging engine includes ascavenging passage communicating with a crank chamber and a combustionchamber, and this scavenging passage is charged with air. In ascavenging stroke, the air in the scavenging passage is first suppliedto the combustion chamber, and an air-fuel mixture is then supplied fromthe crank chamber through the scavenging passage to the combustionchamber. The rotary carburetor disclosed in Patent Document 5 has twopassages formed in a rotatable valve main body. One is a passagegenerating the air-fuel mixture, and the nozzle described above isarranged in this intra-carburetor air-fuel mixture passage. The other isa passage for supplying air to the scavenging passage.

PRIOR ART DOCUMENTS

-   Patent Document 1: Japanese Laid-Open Patent Publication No.    2016-133075 (counterpart US 2016/0208685 A1)-   Patent Document 2: Japanese Laid-Open Patent Publication No.    2016-142271 (counterpart US 2016/0230704 A1)-   Patent Document 3: Japanese Laid-Open Patent Publication No.    2001-73878-   Patent Document 4: Japanese Laid-Open Patent Publication No.    2005-146980-   Patent Document 5: U.S. Pat. No. 7,261,281 B2

In increasingly computerized portable engine working machines, thesolenoid valve described above is effectively used for control of a fuelsupply amount (Patent Documents 1, 2). This solenoid valve is interposedin a fuel supply passage in a carburetor. When the solenoid valveoperates, an amount of fuel passing through the fuel supply passage iscontrolled. Consequently, this controlled amount of fuel furtheradvances through the intra-carburetor fuel supply passage, and the fuelis supplied through the fuel discharge port to the intra-carburetorair-fuel mixture passage.

U.S. Pat. No. 9,273,658 B2 discloses that a magnet is incorporated in afuel filter of a portable engine working machine to purify fuel with themagnet. As can immediately be understood from “purification of fuel by amagnet”, the electromagnetic force of the solenoid valve attractsparticles such as iron powder contained in the fuel. The particlesaccumulating in the solenoid valve cause partial or complete blockage ofthe intra-carburetor fuel supply passage.

As a result of studies for achieving further responsiveness, the presentinventors focused attention on a combination of a nozzle of a rotarycarburetor and a needle inserted into this nozzle. Specifically, thecontrol of the fuel supply amount through relative displacement in theaxial direction between the needle and the nozzle is provided bydirectly controlling a fuel discharge port. Focusing attention on thisdirect fuel control mechanism, the present inventors conceived thepresent invention.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a portable engineworking machine capable of improving responsiveness of fuel supplycontrol and a rotary carburetor incorporated therein.

According to a viewpoint of the present invention, the technical problemdescribed above is solved by providing a portable engine working machinecomprising:

a rotary carburetor disposed in an intake passage of the portable engineworking machine;

a rotatable valve main body included in the rotary carburetor,mechanically coupled to a throttle trigger operated by an operator, androtated by an operation of the throttle trigger to change a throttleopening degree;

a nozzle disposed on an axis of the rotatable valve main body andincluding a fuel discharge port supplying a fuel to an intra-carburetorair-fuel mixture passage;

a needle disposed coaxially with the nozzle and disposed with a portioninserted into the nozzle, the needle being displaced relative to thenozzle to change an effective area of the fuel discharge port so as tocontrol an amount of a fuel discharged from the fuel discharge port;

an electric motor for displacing the needle along an axis; and

a drive mechanism component interposed between the electric motor andthe needle and converting a rotational movement of the electric motorinto a linear movement.

According to another viewpoint of the present invention, the technicalproblem described above is solved by providing a rotary carburetordisposed in an intake passage of a portable engine working machinecomprising:

a rotatable valve main body mechanically coupled to a throttle triggeroperated by an operator, the rotatable valve main body rotated by anoperation of the throttle trigger to change a throttle opening degree;

a nozzle disposed on an axis of the rotatable valve main body andincluding a fuel discharge port supplying a fuel to an intra-carburetorair-fuel mixture passage;

a needle disposed coaxially with the nozzle and disposed with a portioninserted into the nozzle, the needle being displaced relative to thenozzle to change an effective area of the fuel discharge port so as tocontrol an amount of a fuel discharged from the fuel discharge port;

an electric motor for displacing the needle along an axis; and

a drive mechanism component interposed between the electric motor andthe needle and converting a rotational movement of the electric motorinto a linear movement.

Since the amount of the fuel discharged from the fuel discharge port isdirectly controlled by the needle inserted into the nozzle, the presentinvention not only provides excellent responsiveness but also has norisk that iron powder etc. contained in fuel is locally accumulated inan intra-carburetor fuel supply passage due to an electromagnetic forceas in the solenoid valve.

Effects and further objects of the present invention will becomeapparent from the following detailed description of preferredembodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a rotary carburetor mounted on anengine working machine of a first embodiment.

FIG. 2 shows an exploded perspective view of the rotary carburetor ofFIG. 1.

FIG. 3 shows a longitudinal sectional view of the rotary carburetor ofFIG. 1.

FIG. 4 shows a schematic view for explaining a structure of a rotarycarburetor mounted on an engine working machine of a second embodiment.

FIG. 5 shows a view for explaining one method of zero-point adjustmentof a motor and a zero-point is defined as a point at which a needlecollides with a plane serving as a reference of a carburetor main bodywhen the needle is extended.

FIG. 6 shows a view for explaining another method of zero-pointadjustment of a motor and a zero-point is defined as a point at which astep portion between a drive mechanism component and the needle collideswith an end surface of a nozzle.

FIG. 7 shows a view for explaining the other method of zero-pointadjustment of a motor and a zero-point is defined as a limit value,i.e., a point at which the needle can no longer be raised, when theneedle is continuously raised by an electric motor.

FIG. 8 shows a flowchart for explaining one zero-point adjustmentmethod.

FIG. 9 shows a diagram for explaining the zero-point adjustment shown inFIG. 8 performed during deceleration.

FIG. 10 shows a flowchart for explaining the other zero-point adjustmentmethod.

FIG. 11 shows a diagram for explaining the zero-point adjustment shownin FIG. 10 performed during acceleration.

FIG. 12 shows a diagram for explaining one modification example ofarrangement of a position sensor.

FIG. 13 shows a diagram for explaining another modification example ofarrangement of the position sensor.

FIG. 14 shows a diagram for explaining another modification example ofarrangement of the position sensor.

FIG. 15 shows a diagram for explaining the other modification example ofarrangement of the position sensor.

FIG. 16 shows a diagram for explaining an outline of a rotary carburetoraccording to the present invention preferably applicable to a stratifiedscavenging engine.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings. A rotary carburetor of theembodiments is incorporated in a portable engine working machine.Typical examples of the portable engine working machine include chainsaws and bush cutters. Although a two-stroke engine is a typical exampleof an engine, the engine may obviously be a four-cycle engine.

First Embodiment (FIGS. 1 to 3)

FIGS. 1 to 3 show a rotary carburetor mounted on a portable engineworking machine according to a first embodiment of the presentinvention. FIG. 1 is a perspective view. FIG. 2 is an explodedperspective view. FIG. 3 is a longitudinal sectional view.

Referring to FIGS. 1 to 3, a shown rotary carburetor 100 has acarburetor main body 2, and a columnar valve main body 4 is received inan axis-rotatable manner in the carburetor main body 2. This valve mainbody 4 is not displaced in the axial direction.

As in the prior art, the carburetor main body 2 has two openings 2 aopposed to each other. The cylindrical valve main body 4 has onethrough-hole 4 a. This through-hole 4 a forms an intra-carburetorpassage 6 together with the two openings 2 a, and an air-fuel mixture isgenerated in the intra-carburetor passage 6. Therefore, thisintra-carburetor passage 6 will be referred to as an “intra-carburetorair-fuel mixture passage” in the following description.

The axial rotation of the cylindrical valve main body 4 controls aneffective passage cross-sectional area of the intra-carburetor air-fuelmixture passage 6, i.e., a throttle valve opening degree as in the priorart.

The rotary carburetor 100 has a nozzle 8 fixed to the carburetor mainbody 2 as in the prior art (FIG. 3). The nozzle 8 extends upward on theaxis of the valve main body 4 and penetrates the valve main body 4 intothe carburetor air-fuel mixture passage 6. Therefore, the valve mainbody 4 is rotatable around the stationary nozzle 8. A tip portion of thenozzle 8 is provided with a fuel discharge port 8 a in a circumferentialsurface thereof (FIG. 3) and, when fuel is discharged from the fueldischarge port 8 a, the air-fuel mixture is generated in theintra-carburetor air-fuel mixture passage 6 and this air-fuel mixture issupplied to a crank chamber as in the prior art.

A portion of a needle 10 is inserted in the nozzle 8 as in the priorart. Therefore, the needle 10 is arranged on the axis of the valve mainbody 4. In other words, the needle 10 is coaxial with the nozzle 8. Theeffective area of the fuel discharge port 8 a is defined by a tipportion of the needle 10, and the needle 10 is vertically moved tocontrol an amount of fuel supplied through the fuel discharge port 8 ato the intra-carburetor air-fuel mixture passage 6.

The needle 10 is provided with a drive mechanism component 12 verticallydisplacing the needle 10. The drive mechanism component 12 includes aconversion mechanism using, for example, a screw for converting arotational movement to a linear movement. An electric motor 14 (FIG. 1)is coupled to the drive mechanism component 12. A typical example of theelectric motor is a stepping motor.

Reference numeral 18 shown in FIG. 2 denotes a return spring, andreference numeral 20 denotes a cover member. The carburetor main body 2receiving the valve main body 4 is closed by the cover member 20. Thereturn spring 18 is interposed between the cover member 20 and the valvemain body 4.

The valve main body 4 has a cylindrical throttle shaft 22 extendingupward, and this throttle shaft 22 extends upward through the covermember 20. The throttle shaft 22 is rotatable relative to the covermember 20. The outer circumferential surface of the throttle shaft 22has a non-circular irregular cross-sectional shape (FIG. 2).

A throttle lever 24 and a position sensor 26 are arranged around thethrottle shaft 22. A case of the position sensor 26 has a ring shape andis arranged coaxially with the throttle shaft 22. The case of theposition sensor 26 has a shape surrounding at least a portion of thecircumference of the throttle shaft 22 and is fixed to the cover member20 by a fixing member 28 (FIG. 3) surrounding an upper end portion ofthe drive mechanism component 12, and first bolts 30. Although thefixing member 28 is not shown in FIG. 2, the drive mechanism component12 is fastened to the fixing member 28 by second bolts 32 and the drivemechanism component 12 is received in the hollow throttle shaft 22.

In the throttle lever 24, an opening receiving the throttle shaft 22 hasan irregular shape complementary to the throttle shaft 22, so that thethrottle lever 24 is integrated with the throttle shaft 22, i.e., thevalve main body 4. Therefore, the throttle lever 24 is coupled to thethrottle shaft 22 in a relatively non-rotatable manner. The throttlelever 24 is mechanically linked through a wire (indicated by “W” in FIG.4) to a throttle trigger (indicated by “Tt” in FIG. 4) operated by anoperator. When the operator operates the throttle trigger Tt, themovement of the throttle lever 24 interlocking with this operationcauses the valve main body 4 to rotate around an axis so as to controlthe passage effective cross-sectional area of the intra-carburetorair-fuel mixture passage 6, i.e., the throttle valve opening degree.

The ring-shaped position sensor 26 arranged around the throttle shaft 22can detect the rotational position of the throttle lever 24. Therefore,the throttle valve opening degree of the rotary carburetor 100 canlinearly be detected by the position sensor 26. In FIG. 1, referencenumeral 34 denotes a control unit with a memory 36, and referencenumeral 38 denotes a sensor for detecting a rotation speed of an engine.

The throttle valve opening degree detected by the position sensor 26 isapplied to the control of the fuel supply amount together with theengine rotation speed, for example. Specifically, the position sensor 26can sense that the valve main body 4 is (i) located at an idle position,(ii) located at a fully open position, and (iii) located at a partialposition, and (iv) a rotational speed of the valve main body 4, i.e., athrottle valve opening degree change speed, (v) a rotation amount of thevalve main body 4, i.e., a throttle valve opening degree change amount,etc. These are applied to the control unit 34 to control the electricmotor 14 (FIG. 1), i.e., the needle 10, so that the effective openingarea of the fuel discharge port 8 a of the nozzle 8 can directly becontrolled. Thus, the optimization of fuel supply can be achieved withexcellent responsiveness.

This optimization of fuel supply is achieved without using a solenoidvalve as in the prior art and therefore has no risk of causing theproblem of using the solenoid valve, i.e., the problem that iron powderin fuel is accumulated and consequently clogs an intra-carburetor fuelpassage.

The position sensor 26 only needs to detect the rotation of the throttleshaft 22 within the rotational range of the valve main body 4 andtherefore can have a shape defined by this detection range; however, inthe case of the position sensor 26 having a ring shape, this sensor iseasily arranged around the throttle shaft 22 of the valve main body 4,so that the rotary carburetor 100 including the position sensor 26 canbe made compact.

Second Embodiment (FIG. 4)

FIG. 4 shows a rotary carburetor 200 mounted on a portable engineworking machine of a second embodiment. In the description of the rotarycarburetor 200 shown in FIG. 4, the same elements as those of thefirst-mentioned rotary carburetor 100 are denoted by the same referencenumerals. FIG. 4 is a view for explaining a main part of the rotarycarburetor 200. The main part of the rotary carburetor 200 will bedescribed with reference to FIG. 4. Although not shown in FIG. 4, thering-shaped position sensor 26 described in the first embodiment may bedisposed in the rotary carburetor 200.

The valve main body 4 received in the carburetor main body 2 describedwith reference to FIG. 1 etc. rotates around an axis and is verticallydisplaced along the axis. Therefore, the rotary carburetor 200 has avertical drive mechanism vertically moving the valve main body 4. Thevertical displacement of the valve main body 4 is accompanied by avertical motion of the needle 10. In other words, the valve main body 4and the needle 10 rotate together and are vertically displaced together.On the other hand, the nozzle 8 is fixed to the carburetor main body 2and therefore is not vertically displaced. The rotation of the valvemain body 4 is induced by the throttle lever 24 mechanicallyinterlocking through the wire W with an operator's operation of thethrottle trigger Tt. The rotation of the valve main body 4 isaccompanied by a vertical motion thereof and is also accompanied by avertical motion of the needle 10. The vertical displacement of theneedle 10 changes the effective area of the fuel discharge port 8 aopened in the circumferential surface of the nozzle 8. Specificdescription will hereinafter be made with reference to FIG. 4.

Referring to FIG. 4, the valve main body 4 has a cam 204 on an end facethereof, which is a lower end surface in this embodiment, and a camsurface 204 a of the cam 204 is engaged with a support pin 206 fixed tothe carburetor main body 2. When the valve main body 4 rotates aroundthe axis, the cam 204 vertically displaces the valve main body 4. Thisvertical drive mechanism is the same as the rotary carburetor of PatentDocument 4 cited above.

The cylindrical throttle shaft 22 extending upward from the valve mainbody 4 extends upward through the cover member 20 and has an upper endfixed to the throttle lever 24.

The rotary carburetor 200 of the second embodiment includes the needle10 and the drive mechanism component 12 vertically driving the needle 10with the electric motor 14 (FIG. 1), and the drive mechanism component12 has an upper end portion fixed to the throttle lever 24 by the fixingmember 28 and bolts 202.

When the operator operates the throttle trigger Tt, this operation istransmitted through the wire W to the throttle lever 24, and thethrottle lever 24 rotates. When the throttle lever 24 rotates, thethrottle shaft 22 rotates. Therefore, the valve main body 4 rotates.This causes a change in the passage effective cross-sectional area ofthe intra-carburetor air-fuel mixture passage 6 (FIG. 1), i.e., thethrottle opening degree.

When the valve main body 4 rotates, the valve main body 4 is displacedupward or downward by the cam 204. This displacement is transmittedthrough the throttle shaft 22 and the throttle lever 24 to the drivemechanism component 12 and the needle 10 is displaced upward or downwardtogether with the drive mechanism component 12. On the other hand, sincethe nozzle 8 is fixed to the carburetor main body 2 (FIG. 3), theeffective area of the fuel discharge port 8 a of the nozzle 8 changesdue to the displacement of the needle 10.

In the rotary carburetor 200 shown in FIG. 4, the effective area of thefuel discharge port 8 a is mechanically controlled in conjunction withthe operator's trigger operation. This can be used as a basis forapplying electronic control using the electric motor 14. Therefore, acontrol amount of the optimum control of the fuel supply amount usingthe electric motor 14 has a corrective meaning, so that a small controlamount thereof is sufficient. When the control amount is smaller, theresponsiveness of the control becomes better, and consequently, theresponsiveness of the optimum control of the fuel supply amount canfurther be improved.

Zero-Point Adjustment (FIGS. 5 to 11):

By adjusting the origin of upward or downward displacement of the needle10 based on the rotation of the electric motor (stepping motor) 14,i.e., adjusting a zero-point, the accuracy of the fuel supply amountcontrol can be ensured. This zero-point adjustment is performed by usinga predetermined reference plane included in the rotary carburetors 100,200 and the electric motor 14. Three examples of the zero-pointadjustment will be described with reference to FIGS. 5 to 7.

FIG. 5 is a view for explaining one example of the zero-point adjustmentwhen the carburetor main body 2 has a concave portion under the needle10 and a bottom surface 42 of this concave portion is used as thereference plane. Specifically, the zero-point adjustment is performed bydefining as the origin, i.e., the zero-point, a point at which theneedle 10 collides with the bottom surface 42 of the concave portion andcauses the stepping motor 14 to step out when the needle 10 is deviatedfrom a displacement range of control of the needle 10, i.e., a movablecontrol range of the needle 10, and further displaced downward.

FIG. 6 shows another example in which the drive mechanism component 12is deviated from a movable control range of the drive mechanismcomponent 12 and further displaced downward to cause a step portion 12 abetween the needle 10 and the drive mechanism component 12 to collidewith an upper end surface of the nozzle 8. Therefore, FIG. 6 shows anexample of the zero-point adjustment performed by defining as theorigin, i.e., the zero-point, a point at which the step portion 12 aabuts on the nozzle 8. Thus, the example of FIG. 6 is an example inwhich the upper end surface of the nozzle 8 is used as the referenceplane.

FIG. 7 shows the other example in which the zero-point is set as a pointat which the needle 10 can no longer move upward, i.e., a point at whichthe needle 10 can no longer be retracted, when the needle 10 isdisplaced (retracted) upward to the maximum. Specifically, in FIG. 7,the zero-point adjustment is performed by defining as the origin, i.e.,the zero-point, a point at which the needle 10 can no longer move upwardwhen the needle 10 is deviated from the movable control range of theneedle 10 and further displaced upward.

The zero-point adjustment is preferably performed when the operationstate of the engine is a predetermined operation state. The examples ofFIGS. 5 and 6 include a step of completely closing the fuel dischargeport 8 a with the needle 10 and are therefore suitably performed at thetime of deceleration. The example of FIG. 7 includes a step ofcompletely opening the fuel discharge port 8 a and is therefore suitablyperformed at the time of acceleration.

The zero-point adjustment during deceleration will be described withreference to FIGS. 8 and 9. In FIG. 8, it is determined at step S10whether a throttle operation is in a deceleration state. Thisdetermination is made based on the output of the position sensor 26. Atnext step S11, it is determined whether the throttle opening degree isat the idle position. This determination is also made based on theoutput of the position sensor 26. At next step S12, it is determinedwhether a change amount −Δα of the throttle operation is equal to orgreater than a first threshold value. If all the steps of steps S10 toS12 are YES, it is determined that the current operation statecorresponds to a region with zero-point adjustment shown in FIG. 9, andthe zero-point adjustment is performed at step S13. This zero-pointadjustment is performed by the method described with reference to FIG. 5or 6. For example, in the example of FIG. 5, the needle 10 is displaceddownward to collide with the bottom surface 42 of the concave portion.At next step S14, the position of the needle 10 abutting on the bottomsurface 42 is set as the zero-point, and this value on the memory 36(FIG. 1) is updated to zero.

The zero-point adjustment during acceleration will be described withreference to FIG. 10. In FIG. 10, it is determined at step S20 whether athrottle operation is in an acceleration state. This determination ismade based on the output of the position sensor 26 through the controlunit 34 (FIG. 1). At next step S21, it is determined whether thethrottle opening degree is at the fully open position. Thisdetermination is also made based on the output of the position sensor26. At next step S22, it is determined whether a change amount. Δα ofthe throttle operation is equal to or greater than a second thresholdvalue. If all the steps of steps S20 to S22 are YES, it is determinedthat the current operation state corresponds to a region with zero-pointadjustment shown in FIG. 11, and the zero-point adjustment is performedat step S23. This zero-point adjustment is performed by the methoddescribed with reference to FIG. 7. Specifically, the needle 10 isdisplaced upward to move the needle 10 to a position at which the needle10 can no longer be displaced. At next step S24, the zero-point is setto the position at which the needle 10 can no longer be displacedupward, and this value on the memory 36 (FIG. 1) is updated to zero.

Regarding the arrangement of the position sensor 26 in the first andsecond embodiments, since the position sensor 26 is for the purpose ofdetecting the throttle opening degree, the arrangement position of theposition sensor 26 is arbitrary as long as this purpose is achieved, asillustratively shown in FIGS. 12 to 15.

As shown in FIG. 12, a drive gear or drive roller 300 may be disposed onan output shaft of the electric motor 14. The position sensor 26 detectsthe rotation of the drive gear or drive roller 300 through anintermediate gear or roller 302 interposed between the position sensor26 and the drive gear or drive roller 300.

As shown in FIG. 13, the position sensor 26 may be attached to thecarburetor main body 2, and this position sensor 26 may be arranged at aposition where the rotation of the valve main body 4 can be detected.

As shown in FIG. 14, the position sensor 26 may be disposed between thecarburetor main body 2 and the cover member 20 to detect the rotation ofthe valve main body 4.

As shown in FIG. 15, the valve main body 4 may be provided with acylindrical part 306 extending downward from the lower surface thereof,and the position sensor 26 may be disposed around the cylindrical part306.

Although the first and second embodiments and modification examplesthereof have been described, the present invention is not limited to arotary carburetor having the one through-hole 4 a (the intra-carburetorair-fuel mixture passage 6) in the valve main body 4. The presentinvention is also applicable to the stratified scavenging rotarycarburetor disclosed in Patent Document 5 (U.S. Pat. No. 7,261,281 B2).FIG. 16 shows the valve main body 4 included in a stratified scavengingrotary carburetor 350 of a third embodiment. It should be understoodthat the other constituent elements of this stratified scavenging rotarycarburetor 350 is substantially the same as the first and secondembodiments. Referring to FIG. 16, the valve main body 4 has an airpassage 352 in addition to the intra-carburetor air-fuel mixture passage6. Air is supplied through this air passage 352 to a scavenging passageof a stratified scavenging engine. This scavenging passage is denoted byreference numerals 13, 14 in FIG. 2 of US 2014/0360467 A1 and U.S. Pat.No. 8,166,931 B2. In U.S. Pat. No. 8,833,316 B2, a specificconfiguration of the scavenging passage is disclosed in FIG. 3(reference numeral 34). These disclosures of US 2014/0360467 A1, U.S.Pat. No. 8,166,931 B2, and U.S. Pat. No. 8,833,316 B2 are incorporatedin this description. The air supplied to the scavenging passage issupplied as “leading air” to a combustion chamber of a two-stroke enginein a scavenging stroke thereof.

EXPLANATIONS OF LETTERS OR NUMERALS

-   100 rotary carburetor of first embodiment-   200 rotary carburetor of second embodiment-   350 rotary carburetor of third embodiment-   2 carburetor main body-   4 valve main body (rotating member)-   6 intra-carburetor air-fuel mixture passage-   8 nozzle-   8 a fuel discharge port-   10 needle-   12 drive mechanism component-   14 electric motor-   22 hollow throttle shaft-   24 throttle lever (rotating member)-   26 position sensor-   34 control unit-   36 memory-   204 a cam surface-   300 drive gear or drive roller

What is claimed is:
 1. A portable engine working machine comprising: arotary carburetor disposed in an intake passage of the portable engineworking machine; a rotatable valve main body included in the rotarycarburetor, mechanically coupled to a throttle trigger operated by anoperator, and rotated by an operation of the throttle trigger to changea throttle opening degree; a nozzle disposed on an axis of the rotatablevalve main body and including a fuel discharge port supplying a fuel toan intra-carburetor air-fuel mixture passage; a needle disposedcoaxially with the nozzle and disposed with a portion inserted into thenozzle, the needle being displaced relative to the nozzle to change aneffective area of the fuel discharge port so as to control an amount ofa fuel discharged from the fuel discharge port; an electric motor fordisplacing the needle along an axis; a drive mechanism componentinterposed between the electric motor and the needle and converting arotational movement of the electric motor into a linear movement; and acontrol unit for zero-point adjustment adjusting an origin of theelectric motor, wherein the control unit sets as the origin a positionat which the needle is no longer displaceable upward or downward whenthe electric motor is driven out of a control range of displacement ofthe needle for controlling an amount of the fuel.
 2. The portable engineworking machine of claim 1, wherein the rotatable valve main body has ahollow throttle shaft extending on the axis of the rotatable valve mainbody, and a throttle lever coupled to the throttle shaft in a relativelynon-rotatable manner and mechanically coupled to the throttle trigger,and wherein the drive mechanism component is received in the throttleshaft.
 3. The portable engine working machine of claim 2, furthercomprising a vertical drive mechanism vertically displacing therotatable valve main body when the rotatable valve main body is inrotational motion.
 4. The portable engine working machine of claim 3,wherein the vertical drive mechanism includes a cam surface disposed onan end surface of the rotatable valve main body.
 5. The portable engineworking machine of claim 2, further comprising a drive gear or driveroller rotating in conjunction with an operation of the throttletrigger, and a position sensor for detecting a rotation of the drivegear or drive roller, wherein a throttle opening degree is detected bythe position sensor through an intermediate gear or roller interposedbetween the position sensor and the drive gear or drive roller.
 6. Theportable engine working machine of claim 2, further comprising aposition sensor disposed to surround a rotating member rotating inconjunction with an operation of the throttle trigger, wherein athrottle opening degree is detected by the position sensor.
 7. Theportable engine working machine of claim 6, wherein the rotating memberis the throttle lever.
 8. The portable engine working machine of claim6, wherein the rotating member is the rotatable valve main body.
 9. Arotary carburetor disposed in an intake passage of a portable engineworking machine comprising: a rotatable valve main body mechanicallycoupled to a throttle trigger operated by an operator, the rotatablevalve main body rotated by an operation of the throttle trigger tochange a throttle opening degree; a nozzle disposed on an axis of therotatable valve main body and including a fuel discharge port supplyinga fuel to an intra-carburetor air-fuel mixture passage; a needledisposed coaxially with the nozzle and disposed with a portion insertedinto the nozzle, the needle being displaced relative to the nozzle tochange an effective area of the fuel discharge port so as to control anamount of a fuel discharged from the fuel discharge port; an electricmotor for displacing the needle along an axis; a drive mechanismcomponent interposed between the electric motor and the needle andconverting a rotational movement of the electric motor into a linearmovement; and a control unit for zero-point adjustment adjusting theorigin of the electric motor, wherein the control unit sets as theorigin a position at which the needle is no longer displaceable upwardor downward when the electric motor is driven out of a control range ofdisplacement of the needle for controlling an amount of the fuel. 10.The rotary carburetor of claim 9, wherein the rotatable valve main bodyhas a hollow throttle shaft extending on the axis of the rotatable valvemain body, and a throttle lever coupled to the throttle shaft in arelatively non-rotatable manner and mechanically coupled to the throttletrigger, and wherein the drive mechanism component is received in thethrottle shaft.
 11. The rotary carburetor of claim 10, furthercomprising a vertical drive mechanism vertically displacing therotatable valve main body when the rotatable valve main body is inrotational motion.
 12. The rotary carburetor of claim 11, wherein thevertical drive mechanism includes a cam surface disposed on an endsurface of the rotatable valve main body.
 13. The rotary carburetor ofclaim 9, wherein the rotary carburetor is applied to a stratifiedscavenging engine, and wherein the rotatable valve main body furtherincludes an air passage supplying air to a scavenging passage of thestratified scavenging engine.